Systems for projecting images onto screens are known in the art. Systems based on incandescent or arc lamps are often used in projection displays. However, such sources have limited lifetimes and poor output stability over the life of the light source. In addition, spectral components, such as UV and IR, that are not needed for display purposes must be filtered out of the emissions to avoid damage to other components or the viewer's eyes.
Sources based on combinations of Light Emitting Diodes (LEDs) or semiconductor lasers that emit red, green and blue light can be used as a solid state light source. Since LED and laser emissions are typically spectrally narrow band light sources, the use of color filters for the display can be omitted. Also the optical output of semiconductor lasers and LEDs can typically be electrically modulated at a rate that is sufficient for a time sequential color display. Furthermore most solid state lasers emit linear polarized light which further improves the efficiency of light production and simplifies the optics in projection systems.
Unfortunately, individual solid state light sources do not provide sufficient brightness for many projection display applications; hence, arrays of LEDs must be utilized to obtain sufficient output. In order to provide a full color display, the solid state light source has to include emitters of at least three different colors forming a mosaic emitter pattern. Assuming color independent optical losses in a display application, the area of the LED array that is covered by each kind of emitter has to be adjusted to provide white light when all emitters are active. Hence, the radiance of the LED array is dominated by the lowest occurring radiance of emitters of any color. The composition of the LED array can be adjusted to compensate for color dependent losses in the display application.
Present LED technology provides devices with higher radiance in the red and blue spectral regions than for green light. Further, the spectrum of these high power green LEDs shifts to shorter wavelengths with increasing drive current or output power. This wavelength shift creates color distortions in the image if the LED output power is amplitude modulated in order to provide gray scaling in the image.
Broadly, it is the object of the present invention to provide an improved solid state light source that emits in the green region of the optical spectrum.
It is a further object of the present invention to provide a green light source having a spectral output that does not shift when the output of the light source is altered.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.